In the manufacture of semiconductor devices using ion implantation, it is desirable to control the beam incidence angle, because the implantation depth is a function of the beam incidence angle as a result of the well-known channeling effect. Further, it is sometimes desirable to provide beam incidence angles other than perpendicular to the substrate plane for reasons related to the geometry of the device being manufactured. A description of ion implantation techniques utilizing tilted implantation steps can be found in U.S. Pat. No. 5,696,382 issued Dec. 9, 1997 to Kwang, and U.S. Pat. No. 5,909,622 issued Jun. 1, 1999 to Kadosh et al. As device geometries are reduced, semiconductor manufacturers increasingly require improved accuracy in measuring and controlling beam incidence angle in ion implanters. Further, semiconductor device manufacturers typically require the use of parallel scan techniques, wherein the ion beam has a constant angle of incidence on the semiconductor wafer.
Prior art ion beam incidence angle measurement involves actual angle measurement of the wafer chuck or supporting hardware. In most cases, measurement is not made in real time but is done only at maintenance intervals. Angle measurement usually employs mechanical devices, such as squares, levels, and plumb bobs. Laser beams have also been used to measure alignment. In this approach, measurements are not from semiconductor wafer to ion beam, but rather from supporting hardware to supporting hardware. There is no guarantee that the ion beam, an energetic entity sensitive to magnetic and electric fields, bears any fixed angular relationship to any mechanical hardware. Consequently angular measurements are limited to accuracies of approximately ±1 degree.
U.S. Pat. No. 5,180,918, issued Jan. 19, 1993 to Isobe, describes a method and apparatus for measuring ion beam collimation, shaping the ion beam and controlling scanning thereof. The method utilizes a time-dependent change in the scanning position of the ion beam at both an upstream and a downstream location at mutually corresponding times.
A real time beam angle incidence monitor is described in copending application of G. K. Larsen et. al, entitled “In Situ Ion Beam Incidence Angle and Beam Divergence Monitor”, U.S. patent application Ser. No. 10,050,636, filed Jan. 16, 2002, the entirety of which is incorporated herein by reference. The measurement device uses an aperture and variable resistor to measure implant angle. While this method provides a real time method of angular measurement of wafer chuck directly to the ion beam (as opposed to the waveguide or other hardware), its use will be limited to regimes dictated by its accuracy.